The activity of serum ribonuclease (R Nase) was measured in 34 patients with far advanced multiple myeloma and differing degrees of renal insufficiency. RNase levels are not a biomarker for the extent of plasma cell tumor. However, a strong inverse correlation was observed between the degree of renal impairment (expressed as creatinine clearance) and serum RNase levels. Time course studies of individual multiple myeloma patients indicated, without exception, that RNase levels reflect the status of renal function and not the extent of myeloma. Levels of this enzyme, therefore, are not of use in determining the response of tumor to treatment. At present, determinations of RNase levels in patients with leukemia or other malignancies are in progress with the intent of proving that RNase levels are not a biomarker for any of these types of cancer, and also that increases above normal values of levels of this enzyme are an early indicator of impending renal distress.